The present invention generally relates to a method and system for combusting hydrocarbon fuels with resulting ultra-low emissions over a wide range of power levels, fuel properties and ambient operating conditions.
The conventional gas turbine combustor, as used in a gas turbine power generating system, requires a mixture of fuel and air which is ignited and combusted uniformly. Generally, the fuel injected from a fuel nozzle into the inner tube of the combustor is mixed with air for combustion, fed under pressure from the air duct, ignited by a spark plug and combusted. The gas that results is lowered to a predetermined turbine inlet temperature by the addition of cooling air and dilutent air, then injected through a turbine nozzle into a gas turbine.
It is well known within the art that exhaust gases produced by combusting hydrocarbon fuels can contribute to atmospheric pollution. This occurrence is attributed to the development of localized high temperature zone, which can exceed 2,000xc2x0 C. Exhaust gases typically contain many undesirable pollutants such as nitric oxide (NO) and nitrogen dioxide (NO2), which are frequently grouped together as Nitrogen Oxides (NOx), unburned hydrocarbons (UHC), carbon monoxide (CO), and particulates, primarily carbon soot.
Several methods are known in the art to decrease NOx emissions. For example, the formation of fuel-bound NOx can be minimized or avoided entirely by burning a low nitrogen or nitrogen-free fuel. However, burning a low nitrogen fuel does nothing to reduce the formation of thermal or prompt NOx. The formation of thermal NOx can be reduced by operating under uniformly fuel-lean conditions, such as by using a lean diffusion flame or a lean premixed/prevaporized (LPP) system. The excess air used to achieve fuel-lean combustion acts as a diluent to lower flame temperatures, thereby reducing the amount of thermal NOx formed. Prompt NOx can also be reduced by operating under fuel-lean conditions. However, the extent to which thermal and prompt NOx formation can be reduced by fuel-lean combustion may be limited by flame instability that occurs at very lean conditions.
By way of example, Honeywell Air Staged Combustion Systems as used in the ASE120 and ASE50DLE industrial engines are air-staged lean, premixing (LP) combustion systems. Air from the compressor flows over the combustor wall to provide convective cooling and then to at least one three-way air staging valve. Depending on their position, these valves direct air either to the premixers, where the fuel is added and mixed prior to burning in the combustor, or to a bypass manifold which returns the air to the main gas stream downstream of the flame but upstream of the turbine. By modulating the air staging valves the flame temperature can be held substantially constant from no-load to peak conditions. At no-load conditions, a large amount of air is bypassed, while at high power a relatively small amount of air is bypassed allowing the flame temperature to be held close to the ideal for low emissions throughout the power range. An advantage of this system is that all of the compressed air is routed through the turbine, and there is no loss of efficiency as in bleed-type air staging systems. A further advantage is that the combustion system pressure drop remains substantially constant as the air staging valves are modulated. Thus there is little or no impact of the air staging system operation on overall engine efficiency. This provides a system that is accurate and controllable over a wide range of power levels, fuel properties and ambient operating conditions. However, it is not capable of achieving ultra-low emissions.
Catalytic combustion systems, though, are capable of achieving ultra-low emissions. Catalytic combustion systems using a solid phase catalyst are known within the art. However, Catalytic combustion systems are not able to offer the accuracy and controllability of the air staging system over a wide range of power levels, fuel properties and ambient operating conditions.
U.S. Pat. No. 4,040,252, issued to Mosier, a Catalytic Premixing Combustor, discloses a combustor arrangement for a power plant having a tandem, self-regulating arrangement wherein a combustion chamber for burning a fuel-air mixture is placed in line with a catalytic reaction device. This is a representative example of a combination fuel-air combustion chamber and catalytic reaction devices. While such devices are known within the art they are difficult to use, cumbersome, require a great deal of hardware, expensive, and generally require a pre-heater. Pre-heaters are cumbersome and expensive to supply and operate. However, if eliminated in such a systems the catalyst will not activate and this would result in extremely high HC or CO emissions, or the flame will be too lean to sustain. Therefore, a preheater is needed in prior art systems.
Accordingly, what is needed in the art is an easy to use, inexpensive method and system for combusting hydrocarbon fuels that is accurate, controllable, easily adapted to a wide range of power levels, fuel properties and ambient operating conditions, and offers ultra-low emissions without the need for a preheater.
The present invention is directed to an easy to use and inexpensive method and system for combusting hydrocarbon fuels over a wide range of power levels, fuel properties and ambient operating conditions that results in ultra-low emissions.
One aspect of the invention is a system for combusting hydrocarbon fuel, comprising an air supply for supplying air from a compressor to an air inlet, at least one air staging valve, at least one fuel preparation and mixing section for receiving fuel and air directed from the air staging valves, at least one catalyst section for receiving said fuel and air mixture, a combustor, a secondary air stream and an exit for delivering the exiting effluent gas stream generated by the system to a turbine.
The system may be operated in different manners to allow for low and high power operation, as well as according to a controlled schedule that may be programmed. Under low power operation oxidation does not occur in the catalyst section. However, the mixing of the fuel and air in the fuel preparation and mixing section is enhanced by the presence of the catalyst. As the engine power level increases the compressor outlet air temperature will become high enough to activate the catalyst, and partial oxidation reactions will occur.
In one aspect of the present invention, a method of combusting a hydrocarbon fuel is disclosed. According to this method, air is compressed, then divided into at least one air staging valve air stream and at least one secondary air stream. Each air staging valve air stream is divided into at least one bypass flow stream, and at least one primary air stream. The bypass flow stream flows through a bypass manifold, combines with the secondary air stream and the output is an output bypass flow stream. It should be noted that the secondary air stream may consist solely of the control air stream. For instance, at high power, where the temperature profile is important, the output bypass flow stream is low and therefore has little effect. The primary air stream is introduced into a fuel preparation and mixing section, wherein fuel is injected and mixed to form a fuel/air mixture stream, which is introduced into a catalyst section. During certain conditions, which vary, depending on the specific catalyst, no oxidation will occur, but premixing is enhanced by the presence of the catalysts. The product stream that exits the catalyst section is then fed into the combustor. The temperature and composition of the product stream are selected to control simultaneously the amounts of NOx formed in the combustor and the stability of the flame in the combustor, thereby controlling the total amount of NOx in the exit effluent gas stream. Where conditions are desired such that no oxidation occurs in the catalyst section, the air staging valve schedule may be set to yield a flame temperature around 1800 K.
In another aspect of the present invention, a method of combusting a hydrocarbon fuel is disclosed. According to this method, air is compressed, then divided into at least one air staging valve air stream and at least one secondary air stream. Each air staging valve air stream is divided into at least one bypass flow stream, and at least one primary air stream. The bypass flow stream flows through a bypass manifold, combines with the secondary air stream and the output is an output bypass flow stream. This may consist of solely the secondary air stream, or such a low bypass flow stream that it is negligible. Also the control air stream may consist solely of the secondary air stream. The primary air stream is introduced into a fuel preparation and mixing section, wherein fuel is injected and mixed to form a fuel/air mixture stream, which is introduced into a catalyst section, wherein the fuel/air mixture stream is partially oxidized creating a partial oxidation product stream. The partial oxidation product is combined with the control air stream to form an exit effluent gas stream, which exits to the turbine. The temperature and composition of the partial oxidation product stream are selected to control simultaneously the amounts of NOx formed in the combustor and the stability of the flame in the main combustor, thereby controlling the total amount of NOx in the exit effluent gas stream. Where partial oxidation is desired, the air staging valve schedule will be set to yield a flame temperature of around 1700 K.
According to one embodiment, a method of combusting hydrocarbon fuel is disclosed comprising compressing an air stream in a compressor, dividing the air stream into a first air staging valve air stream, a second air staging valve air stream and one secondary air stream, utilizing an air staging valve to controllably divide the first air staging valve air stream into one bypass flow stream and one primary air stream, introducing said bypass flow stream into a bypass manifold. The resulting output bypass flow stream is combined with other air streams and form the control air stream. The primary air stream is then introduced into a fuel preparation and mixing section, wherein fuel is injected and mixed to form a fuel/air mixture stream, which is then introduced into a catalyst section, wherein a catalyst is located and partially oxidizes the fuel by contacting the catalyst mixture with an oxidation catalyst in a catalytic oxidation stage, thereby generating a heat of reaction and a partial oxidation product stream comprising partially oxidized hydrocarbons, carbon monoxide and excess air. The partial oxidation product stream is combusted in a main combustor, at a condition at which appreciable quantities of thermal NOx are not formed, thereby generating an effluent gas stream. The temperature and composition of the partial oxidation product stream are selected to control simultaneously the amounts of NOx formed in the main combustor and the stability of the flame in the main combustor, thereby controlling the total amount of NOx in the exit effluent gas stream. The flame in the main combustor is controlled to a flame temperature between 1700 K and 2000 K by varying the position of the air staging valve. It should be understood by those skilled in the art that a number of different air streams may be combined in different ways in order to form the exit effluent gas stream. By way of example the bypass flow stream, secondary air stream and effluent gas stream may be combined to form an exit effluent gas stream. Also, the secondary air stream and bypass flow stream may be combined to form a control air stream which is then combined with the effluent gas stream to form an exit effluent gas stream.
It is envisioned that, in order to control the air valve schedule, the control system may be programmed to a predetermined schedule. Further, one embodiment is to provide closed loop control, wherein sensors will detect the temperature in the region of the catalyst exit, and provide a control signal to a controller that will adjust the air staging valve schedule to give the appropriate flame temperatures.
These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims.